EB11/Ordnance describes several breech mechanisms based on the interrupted screw principle. Normally the threads of a screw engage continuously with those of a threaded screw box. The problem with a continuous screw breech plug is that it can be time-consuming to tighten and untighten. A 16-inch naval gun might develop a gas pressure of 40,000 psi, necessitating a 1,400 pound plug. (NAVORD).

The basic interrupted screw concept was invented much earlier than you might think. In a musket manufactured at Mayence around 1690, "the muzzle portion turns round one-sixth of a circle, and then pulls out a short distance, liberating the breech-piece, which can be thrown back on a hinge." (Horton 302).

With an interrupted screw, the threads of both are discontinuous, so that there is a screw orientation such that it can be slid into the screw box without engaging. For example, looking down the axis of the box, it might have threading from 12 o'clock to 3 o'clock, and 6 to 9. If so, then the screw in the slide-in orientation would have threading only from 3 to 6 and 9 to 12. Once inserted, such a screw would be given a quarter-turn, and then the threads would be fully engaged. (Wilson 249).

The disadvantage of the classic interrupted screw was that it engaged only along half the circumference and thus, to have the same sealing strength as the continuous screw, would need to be twice as long.

This disadvantage was largely overcome by the Welin stepped interrupted thread. The circumference of the screw is divided into several (2–4) groups. Each group can further be divided circumferentially into several arcs, which progressively increase in diameter, creating a stepped pattern. On the screw, the arcs at the lowest step level are blank, and the other arcs are threaded.

In the disengaged position, a threaded arc on the screw can face a threaded arc on the screw box, provided that the arc on the box is deeper so they don't engage. You slide the screw in and then turn it to engage. With three different threaded diameters, and one smooth, you have threaded engagement for 75 % of the circumference, and with two groups, a one-eighth turn is need to engage. Actually cutting a Welin screw must have been a complete bear.

In canon, there are post-RoF-manufactured breech-loading rifles as of 1634 ( 1634:TBWChap. 27), although in very limited quantity (Chap. 5), but the Americans, in building their first ironclads, deliberately opted for muzzle loaded naval guns because of unspecified resource limitations. (Flint, Weber, 1633, Chap. 4).

Smoothbore versus Rifled

The cannon in use as of the RoF have smoothbore barrels, which means just what it says.

However, the barrel of a firearm may be rifled-given helical grooves-in order to impart a spin to a projectile. The effect would be to gyroscopically stabilize the flight of the projectile.

Rifling was introduced into small arms in the sixteenth century, as we know from a 1563 Swiss ordinance: "For the last few years the art of cutting grooves in the chambers of the guns has been introduced with the object of increasing the accuracy of fire; the disadvantage resulting therefrom to the common marksman has sown discord amongst them. In ordinary shooting matches marksmen are therefore forbidden under a penalty of L10 to provide themselves with rifled arms. Every one is nevertheless permitted to rifle his military weapon and to compete with marksmen armed with similar weapons for special prizes." ( Chamber's Encyclopaedia718). These rifles, apparently, were used to fire balls, since elongated projectiles reportedly were not invented until 1662.

The first rifled artillery pieces were probably those of Cavalli (1846) (Quartstein 45). Both rifles and smoothbores were used in several mid-nineteenth-century naval conflicts, notably the American Civil War, the Second Scheswig War, the Third Italian Independence War, and the Guano War.

Rifling was not a panacea; reloading was more difficult, and range and accuracy were not always improved (the projectiles tumbled if they weren't loaded properly). The metal ("lands") between the grooves can get worn down. Also, during the American Civil War, rifled artillery seemed more prone to burst than muzzle-loading Dahlgrens, and rifled projectiles couldn't gain range by ricochet. (Manucy 17; Jenkins; Schneller). This may explain the Union navy's wartime preference for smoothbores (Heidler 1046), even though in 1859, after comparative testing, the US government had concluded "the era of smoothbore artillery has passed away." (Bell 44).

Even so, there were skeptics. After the Battle of Lissa (1866, Austria vs. Italy), Tegethoff, the Austrian commander, commented, "the lack of results on the part of the enemy have shown that smoothbore guns on the sea have much more value than a rifled one, since a rifle requires for best results at long range a still position, difficult to find on the sea." (Greene 254).

The driving force for the adoption of rifled guns appears to have been not so much increasing effective range but that they could fire an elongated shell, thus one carrying more explosive for a given caliber. (Colomb 340ff). But it took perhaps two decades to perfect heavy rifled cannon (Bell 44; Lewis 65), and Dahlgren smoothbore-armed Civil War vintage monitors were placed on coastal defense duty during the Spanish-American War.

In order to apply spin to the projectile, it must somehow engage the rifling. With small arms, the bullet could be made of lead, which is malleable. There were two problems with making artillery projectiles out of lead. The first was that lead was expensive, and the second was that lead, being soft, would foul the inside of the barrel.

A number of expedients were tested in the nineteenth century. A lead coating on the projectile was introduced by Baron Warhendorff in the 1840s. (Kinard 222). That wouldn't be as expensive as making the whole thing out of lead, but fouling would still be a problem. The British nonetheless used this system with breech loaders.

Whitworth and Lancaster made projectiles with twisted side faces to match a twisted bore, hexagonal for Whitworth, oval for Lancaster. When mass produced, the rounds tended to jam in the bore. The Confederates used some Whitworth rifles.

For rifled muzzle loaders, one had to provide sufficient windage that the projectile could still be rammed down the barrel. One solution (Armstrong, 1854) was to provide the projectiles with studs to engage the grooves of the rifling. The engagement is reliable but the projectile must be studded to match the twist in a particular gun, and the gun cannot have increasing twist. Also, the grooves must be wide and deep to accommodate the studs, and that weakens the gun, whereas the studs increase air resistance to the projectile. (Bruff 303).

If the studs were taller than the depth of the grooves, there would be a clearance between the main body of the projectile and the lands (the uncut portions of the bore between the grooves). (Woolwich 182). Unfortunately, if the studs have clearance, and there's no gas check, then gas escapes and damages the bore.

It was discovered that the copper gas check I mentioned earlier not only reduced the gas loss from windage, it also engaged the rifling. It was used in rifled muzzle loaders, but it was found advantageous to make the grooves shallower and more numerous than in a breech loader.

However, the most successful ploy was to place "a copper 'driving band' into a groove cut around the body of the projectile." (EB11/Ammunition). While the basic concept is in Grantville Literature, there are some serious engineering considerations. We have to figure out what material to make it out of, how thick and long it should be, whether to have one long band or several short ones, where on the projectile body to place it, and how to secure it there. The choices we make, in turn, determine how well it engages the rifling, how much wear it imposes on the bore, and the aerodynamic characteristics of the projectile. (See 1922 EB/ "Ammunition").